When paramecia are placed in homologous immune sera they become immobilized and eventually die. This specific reaction has been shown to be due to a series of protein antigens associated with the cell surface. Fluorescence studies reveal a pattern of Ag-Ab redistribution over the cell surface analogous to the capping phenomenon reported in other cell types. The process of redistribution occurs at the tips of the cilia and frequently results in the agglutination of several cilia into groups. TEM studies on sections through the ciliary bundles of immobilized cells show the axonemes of these agglutinated cilia contained within a single unit membrane. This fusion of ciliary membranes is readily induced by intact immune IgG, but is not induced by Fab fragments prepared from the same IgG. Cross-linking of the Fab fragments with goat anti-rabbit IgG results in immobilization and membrane fusion identical to that seen with intact immune IgG.